A lithographic apparatus is a machine that applies a desired pattern onto a target region of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target region (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target regions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target region is irradiated by exposing an entire pattern onto the target region in one go, and so-called scanners, in which each target region is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In some applications it may be desirable to pattern a substrate in which the upper layer of radiation sensitive material has a number of surfaces at different heights. This may, for example, occur when fabricating three-dimensional integrated circuits, micro electromechanical system (MEMS) devices (for example bio-MEMS devices), micro-fluidic structures or system-in-packaging (SiP) devices. When processing such multi-level substrates the critical dimension of the patterns being exposed may be important.
When a pattern is required to have a small critical dimension, for example less than one micrometer (1 μm), the depth of focus of the projected image may also be small. As such, any features being patterned on surfaces of a substrate that are at different heights, for example in a recess or on a raised portion, may be outside of the focal depth of the projection system when the substrate is normally aligned for exposure (i.e. when the main surface of the substrate lies substantially in line with the focal plane of the projection system). For example a relatively deep recess and or a relatively high raised portion could be fabricated on the surface of a substrate at a height of 100 μm or more above or below the normal surface of the substrate. In contrast, the focal depth of a typical projection system may be in the region of as little as plus or minus 1 μm. Thus, the surface of the recess/raised portion will lie outside of the focus of the projected beam, thereby limiting the accuracy with which an image may be patterned. The term ‘main surface of the substrate’ is intended to mean the surface of the substrate in the absence of a recess or raised portion. This is not intended to be limited to a single surface, and may be a plurality of surfaces which are separated from one another.